Prior assembly of the various pipe portions on board the vessel, and then laying of the pipe on the sea bottom piece by piece by assembling strings together and then immersing the pipe little by little as the strings are assembled together is performed traditionally, in particular when laying undersea pipes in depths of more than 1000 m, or even 2000 m, by a method known as J-laying in which the suspended undersea pipe adopts a curved configuration between its anchor point at the surface via the vessel and its point of contact with the sea bottom, said shape corresponding to a mathematical curve known as a catenary for which the radius of curvature increases regularly going from the bottom up to the surface.
This kind of laying is known as J-laying since the shape of the portion of pipe in suspension between the vessel and the point of contact on the sea bottom is considered, improperly, as being J-shaped. Such J-laying is described in numerous patents, and in particular in the following patents in the name of the Applicant: FR 2 792 991, FR 2 793 540, and FR 2 801 088.
In those patents, a laying vessel is described that is fitted with a J-lay tower that enables the following operations to be performed:
holding stable the emerging top end of the portion of pipe that has already been assembled and that is immersed in suspension;
lowering a new portion of pipe or string, presenting the bottom end of a new portion of pipe or string to said immersing top end of said portion of pipe in suspension;
firmly holding the ends of the portions of pipe for assembling together by welding so that said welding can be performed without risk of damage due to the various movements of the vessel and of the immersed portion of pipe in suspension down to the sea bottom; and finally
welding together said pipe portions.
A difficulty during such J-laying lies in the fact that all of the above-mentioned operations need to be performed at a single location situated on the deck of the vessel, and thus close to the bottom end of the tower, and furthermore, all of the operations need to be performed as quickly as possible, given the extremely high hourly operating cost of the laying vessel.
The pipe that is being assembled is held at the bottom portion of the J-lay tower by a system of external clamps, e.g. as described in the Applicant's patent FR 2 801 088, over a length that cannot exceed a few meters.
When assembly of the pipe has terminated, the top end of the pipe is conventionally lowered to the sea bottom with the help of an abandonment device constituted by an abandonment cable wound on an abandonment winch with the end of the cable fastened to the top end of the pipe that is to be abandoned, as explained below in the detailed description given with reference to FIG. 1. In order to ensure that the tension exerted by the abandonment cable on the top end of the pipe acts substantially along the axial direction of the pipe, and in order to enable the tension in the cable to be taken up at the top end of the tower structure, deflector sheaves are interposed on the path followed by the abandonment cable prior to reaching the abandonment winch.
When laying pipe at great depth, in particular at depths of more than 1000 m, e.g. 2000 m or 3000 m, the loads supported by the abandonment winches and sheaves thus correspond to the weight of the suspended portion of pipe, and these loads may exceed 1000 (metric) tonnes (t). Such abandonment winches and sheaves therefore needs to be dimensioned accordingly so as to withstand such tensions, however for the winch, given the great length of cable required that means a large amount of space and volume is needed on the deck or in the hold of the vessel in order to receive the abandonment winch.
When laying pipes in great depths, the huge nature of the traction means that need to be implemented for the abandonment step is almost unachievable in practice, and in any event would be extremely expensive when using conventional means involving abandonment winches and cables.